Monday 5 December 2016

Video: Scientists crack the case of knuckle cracking

David Kearns

Published 01/12/2015 | 10:56

It might be a sound that makes most people wince but a group of researchers have knuckled down to discover the science behind why joints crack.

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In a new 'cracking' study, scientists from the University of California have recorded simultaneous for the first time the audio and visual evidence of knuckles cracking.

“We were interested in pursuing this study because there’s a raging debate about whether the knuckle-cracking sound results from a bubble popping in the joint or from a bubble being created in the joint,” said Professor Robert D. Boutin, speaking at the annual meeting of the Radiological Society of North America.

Evidence from the study suggests that the “pop” sound from cracking a knuckle or joint is actually caused by the formation of a gas when bones are stretch apart. 

Mr Boutin said: “There have been several theories over the years and a fair amount of controversy about what’s happening in the joint when it cracks.

“We’re confident that the cracking sound is related to the dynamic changes in pressure associated with a gas bubble in the joint.

“As the bones separate, gas in the synovial fluid gathers together, resulting in the sudden formation of bubbles… and with that comes the pop.”

A total of 40 healthy adults, including 17 women and 23 men (age range 18-63), were examined as a part of the study, and ultrasound imaging was used to record them as they attempted to crack the knuckle at the base of each finger, known in medical parlance as the metacarpophalangeal joint (MPJ).

The participants included 30 individuals with a history of habitual knuckle cracking and 10 without.

“What we saw was a bright flash on ultrasound, like a firework exploding in the joint,” said Mr Boutin

“It was quite an unexpected finding.”

But as for which comes first 0 the cracking or the flash of light, Mr Boutin said that more research was needed.

"We found that there was no immediate disability in the knuckle crackers in our study, although further research will need to be done to assess any long-term hazard - or benefit - of knuckle cracking," he added.

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